mq_getattr - Online Linux Manual PageSection : 3
Updated : 2022-12-15
Source : Linux man-pages 6.03
NAMEmq_getattr, mq_setattr − get/set message queue attributes
LIBRARYReal-time library (librt, −lrt)
SYNOPSIS#include <mqueue.h>int mq_getattr(mqd_t mqdes, struct mq_attr *attr);
int mq_setattr(mqd_t mqdes, const struct mq_attr *restrict newattr,
struct mq_attr *restrict oldattr);
DESCRIPTIONmq_getattr() and mq_setattr() respectively retrieve and modify attributes of the message queue referred to by the message queue descriptor mqdes. mq_getattr() returns an mq_attr structure in the buffer pointed by attr. This structure is defined as: struct mq_attr {
long mq_flags; /* Flags: 0 or O_NONBLOCK */
long mq_maxmsg; /* Max. # of messages on queue */
long mq_msgsize; /* Max. message size (bytes) */
long mq_curmsgs; /* # of messages currently in queue */
};The mq_flags field contains flags associated with the open message queue description. This field is initialized when the queue is created by mq_open(3). The only flag that can appear in this field is O_NONBLOCK. The mq_maxmsg and mq_msgsize fields are set when the message queue is created by mq_open(3). The mq_maxmsg field is an upper limit on the number of messages that may be placed on the queue using mq_send(3). The mq_msgsize field is an upper limit on the size of messages that may be placed on the queue. Both of these fields must have a value greater than zero. Two /proc files that place ceilings on the values for these fields are described in mq_overview(7). The mq_curmsgs field returns the number of messages currently held in the queue. mq_setattr() sets message queue attributes using information supplied in the mq_attr structure pointed to by newattr. The only attribute that can be modified is the setting of the O_NONBLOCK flag in mq_flags. The other fields in newattr are ignored. If the oldattr field is not NULL, then the buffer that it points to is used to return an mq_attr structure that contains the same information that is returned by mq_getattr().
RETURN VALUEOn success mq_getattr() and mq_setattr() return 0; on error, −1 is returned, with errno set to indicate the error.
ERRORSEBADF The message queue descriptor specified in mqdes is invalid. EINVAL newattr−>mq_flags contained set bits other than O_NONBLOCK.
ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7). InterfaceAttributeValue mq_getattr(), mq_setattr() Thread safetyMT-Safe
STANDARDSPOSIX.1-2001, POSIX.1-2008.
NOTESOn Linux, mq_getattr() and mq_setattr() are library functions layered on top of the mq_getsetattr(2) system call.
EXAMPLESThe program below can be used to show the default mq_maxmsg and mq_msgsize values that are assigned to a message queue that is created with a call to mq_open(3) in which the attr argument is NULL. Here is an example run of the program: $ ./a.out /testq
Maximum # of messages on queue: 10
Maximum message size: 8192Since Linux 3.5, the following /proc files (described in mq_overview(7)) can be used to control the defaults: $ uname −sr
Linux 3.8.0
$ cat /proc/sys/fs/mqueue/msg_default
10
$ cat /proc/sys/fs/mqueue/msgsize_default
8192
Program source #include <fcntl.h>
#include <mqueue.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <unistd.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
int
main(int argc, char *argv[])
{
mqd_t mqd;
struct mq_attr attr;
if (argc != 2) {
fprintf(stderr, "Usage: %s mq−name\n", argv[0]);
exit(EXIT_FAILURE);
}
mqd = mq_open(argv[1], O_CREAT | O_EXCL, 0600, NULL);
if (mqd == (mqd_t) −1)
errExit("mq_open");
if (mq_getattr(mqd, &attr) == −1)
errExit("mq_getattr");
printf("Maximum # of messages on queue: %ld\n", attr.mq_maxmsg);
printf("Maximum message size: %ld\n", attr.mq_msgsize);
if (mq_unlink(argv[1]) == −1)
errExit("mq_unlink");
exit(EXIT_SUCCESS);
}
SEE ALSOmq_close(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7) 0
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